Past Issue

We published our first issue of Molecular Metabolism in December 2012, and we have been working hard over the last 4 years to establish the journal as a leading voice of the metabolism research community. Since that first issue, we now have published over 400 articles, received an initial impact factor of 5.4, and, just recently, been accepted for direct indexing in the National Library of Medicine (NLM). While we were already indexed in Pub Med Central (PMC), indexing in NLM means that from this issue forward, as soon as an article is accepted, it will appear in PMC rather than appearing after an issue has been paginated (typically 6–12 weeks after acceptance). These may seem like small steps, but in order to disseminate science as quickly as possible and to as wide an audience as possible, these steps actually represent giant strides for the journal toward increased speed, visibility, and impact.

Importantly, 2017 promises to be another exciting year for the journal. In this issue, we are pleased to publish an important commentary by former Harvard Medical School Dean Jeffrey Flier on the irreproducibility of scientific research. This timely paper addresses the widespread occurrence and unquestionable significance of this worrisome “epidemic”, identifies underlying factors that might explain it, and offers suggestions for how best to address the challenge as a scientific community. Later in the year and following the success of our two previous special issues (Metabolic Syndrome: Removing Roadblocks to Therapy in 2014 and Gut Microbiome and Metabolism: From Physiology to Disease in 2016), we will publish a special issue on islet biology. Klaus Kästner and Heiko Lickert are guest editors of this issue, which will feature work from Hans Clevers, Yuval Dor, P-O Berggren, Miriam Cnop, Maike Sander, to name a few.

The success of Molecular Metabolism is undoubtedly owed to you, the editorial board members, reviewers, and submitting authors. We thank you for trusting us and investing so much to support a journal at a time when it had no impact factor and was facing an uncertain future. In return, now we will work twice as hard to provide the metabolism research community with the modern, ultra-fast, high quality peer-reviewed journal it deserves.

During a 40 year career as a biomedical researcher and academic leader, my primary professional goal has been to discover and disseminate new knowledge relevant to biology and health, with my own efforts focused on metabolic physiology and disease. I have done this during a period of dynamic growth of the bioscience enterprise, which has produced remarkable discoveries to illuminate our understanding of human biology and disease while creating numerous benefits for the health and welfare of society. [Hide abstract]

Gut microbiota is now considered to be an important regulator of body weight. With approximately 10 million genes [1], the gut metagenome is certainly hiding numerous mechanisms involved in the control of body weight and the consequent metabolic disease [2]. Among them, bacterial enzymes fermenting non-digestible dietary fibers into the short chain fatty acids (SCFA) acetate, propionate, and butyrate have garnered great attention over the last decade [3]. A major question remains about the identification of molecular targets of SCFA involved in the control of body weight and energy homeostasis. [Hide abstract]

A sizable percentage of the population stands to benefit from elucidating mechanisms linking sleep loss, circadian misalignment, and metabolic disease. In particular, shift work is associated with increased risk for metabolic diseases, including type 2 diabetes, obesity, and metabolic syndrome [1]. These workers also report less sleep per day, and often outside of the biological night. While a reflexive instinct to these discoveries is to encourage more sleep, this may not always be practical for shift working individuals. [Hide abstract]

Objective:
Increased fructose consumption is a contributor to the burgeoning epidemic of non-alcoholic fatty liver disease (NAFLD). Recent evidence indicates that the metabolic hormone FGF21 is regulated by fructose consumption in humans and rodents and may play a functional role in this nutritional context. Here, we sought to define the mechanism by which fructose ingestion regulates FGF21 and determine whether FGF21 contributes to an adaptive metabolic response to fructose consumption.

Methods:
We tested the role of the transcription factor carbohydrate responsive-element binding protein (ChREBP) in fructose-mediated regulation of FGF21 using ChREBP knockout mice. Using FGF21 knockout mice, we investigated whether FGF21 has a metabolic function in the context of fructose consumption. Additionally, we tested whether a ChREBP-FGF21 interaction is likely conserved in human subjects.

Results:
Hepatic expression of ChREBP-β and Fgf21 acutely increased 2-fold and 3-fold, respectively, following fructose gavage, and this was accompanied by increased circulating FGF21. The acute increase in circulating FGF21 following fructose gavage was absent in ChREBP knockout mice. Induction of ChREBP-β and its glycolytic, fructolytic, and lipogenic gene targets were attenuated in FGF21 knockout mice fed high-fructose diets, and this was accompanied by a 50% reduction in de novo lipogenesis a, 30% reduction VLDL secretion, and a 25% reduction in liver fat compared to fructose-fed controls. In human subjects, serum FGF21 correlates with de novo lipogenic rates measured by stable isotopic tracers (R = 0.55, P = 0.04) consistent with conservation of a ChREBP-FGF21 interaction. After 8 weeks of high-fructose diet, livers from FGF21 knockout mice demonstrate atrophy and fibrosis accompanied by molecular markers of inflammation and stellate cell activation; whereas, this did not occur in controls.

Conclusions:
In summary, ChREBP and FGF21 constitute a signaling axis likely conserved in humans that mediates an essential adaptive response to fructose ingestion that may participate in the pathogenesis of NAFLD and liver fibrosis.

Objective:
Fibroblast-growth factor 21 (FGF21) is thought to be important in metabolic regulation. Recently, low protein diets have been shown to increase circulating FGF21 levels. However, when energy contribution from dietary protein is lowered, other macronutrients, such as carbohydrates, must be increased to meet eucaloric balance. This raises the possibility that intake of a diet rich in carbohydrates may induce an increase in plasma FGF21 levels per se. Here we studied the role of dietary carbohydrates on the levels of circulating FGF21 and concomitant physiologic effects by feeding healthy men a carbohydrate rich diet without reducing protein intake.

Methods:
A diet enriched in carbohydrates (80 E% carbohydrate; CHO) and a eucaloric control diet (CON) were provided to nine healthy men for three days. The energy intake during the CHO diet was increased (+75% energy) to ensure similar dietary protein intake in CHO and CON. To control for the effect of caloric surplus, we similarly overfed (+75% energy) the same subjects for three days with a fat-rich diet (78 E% fat; FAT), consisting of primarily unsaturated fatty acids. The three diets were provided in random order.

Objective:
Histone deacetylases are epigenetic regulators known to control gene transcription in various tissues. A member of this family, histone deacetylase 3 (HDAC3), has been shown to regulate metabolic genes. Cell culture studies with HDAC-specific inhibitors and siRNA suggest that HDAC3 plays a role in pancreatic β-cell function, but a recent genetic study in mice has been contradictory. Here we address the functional role of HDAC3 in β-cells of adult mice.

Methods:
An HDAC3 β-cell specific knockout was generated in adult MIP-CreERT transgenic mice using the Cre-loxP system. Induction of HDAC3 deletion was initiated at 8 weeks of age with administration of tamoxifen in corn oil (2 mg/day for 5 days). Mice were assayed for glucose tolerance, glucose-stimulated insulin secretion, and islet function 2 weeks after induction of the knockout. Transcriptional functions of HDAC3 were assessed by ChIP-seq as well as RNA-seq comparing control and β-cell knockout islets.

Objective:
Elevated serum ferritin has been linked to type 2 diabetes (T2D) and adverse health outcomes in subjects with the Metabolic Syndrome (MetS). As the mechanisms underlying the negative impact of excess iron have so far remained elusive, we aimed to identify potential links between iron homeostasis and metabolic pathways.

Methods:
In a cross-sectional study, data were obtained from 163 patients, allocated to one of three groups: (1) lean, healthy controls (n = 53), (2) MetS without hyperferritinemia (n = 54) and (3) MetS with hyperferritinemia (n = 56). An additional phlebotomy study included 29 patients with biopsy-proven iron overload before and after iron removal. A detailed clinical and biochemical characterization was obtained and metabolomic profiling was performed via a targeted metabolomics approach.

Results:
Subjects with MetS and elevated ferritin had higher fasting glucose (p < 0.001), HbA1c (p = 0.035) and 1 h glucose in oral glucose tolerance test (p = 0.002) compared to MetS subjects without iron overload, whereas other clinical and biochemical features of the MetS were not different. The metabolomic study revealed significant differences between MetS with high and low ferritin in the serum concentrations of sarcosine, citrulline and particularly long-chain phosphatidylcholines. Methionine, glutamate, and long-chain phosphatidylcholines were significantly different before and after phlebotomy (p < 0.05 for all metabolites).

Conclusions:
Our data suggest that high serum ferritin concentrations are linked to impaired glucose homeostasis in subjects with the MetS. Iron excess is associated to distinct changes in the serum concentrations of phosphatidylcholine subsets. A pathway involving sarcosine and citrulline also may be involved in iron-induced impairment of glucose metabolism.

Objective:
Dietary supplementation with fermentable carbohydrate protects against body weight gain. Fermentation by the resident gut microbiota produces short-chain fatty acids, which act at free fatty acid receptor 2 (FFAR2). Our aim was to test the hypothesis that FFAR2 is important in regulating the beneficial effects of fermentable carbohydrate on body weight and to understand the role of gut hormones PYY and GLP-1.

Methods:
Wild-type or Ffar2-/- mice were fed an inulin supplemented or control diet. Mice were metabolically characterized and gut hormone concentrations, enteroendocrine cell density measurements were carried out. Intestinal organoids and colonic cultures were utilized to substantiate the in vivo findings.

Results:
We provide new mechanistic insight into how fermentable carbohydrate regulates metabolism. Using mice that lack FFAR2, we demonstrate that the fermentable carbohydrate inulin acts via this receptor to drive an 87% increase in the density of cells that produce the appetite-suppressing hormone peptide YY (PYY), reduce food intake, and prevent diet-induced obesity.

Conclusions:
Our results demonstrate that FFAR2 is predominantly involved in regulating the effects of fermentable carbohydrate on metabolism and does so, in part, by enhancing PYY cell density and release. This highlights the potential for targeting enteroendocrine cell differentiation to treat obesity.

Objective:
Intestinal glucose absorption is orchestrated by specialized glucose transporters such as SGLT1 and GLUT2. However, the role of GLUT2 in the regulation of glucose absorption remains to be fully elucidated.

Methods:
We wanted to evaluate the role of GLUT2 on glucose absorption and glucose homeostasis after intestinal-specific deletion of GLUT2 in mice (GLUT2ΔIEC mice).

Conclusions:
Intestinal GLUT2 modulates glucose absorption and constitutes a control step for the distribution of dietary sugar to tissues. Consequently, metabolic and gut homeostasis are improved in the absence of functional GLUT2 in the intestine, thus mimicking calorie restriction.

Results:
We identified negatively correlated methylation and expression of several obesity-associated genes in our discovery dataset and in silico replicated ETV6 in two independent cohorts. Further, we identified six adipose tissue depot-specific genes (HAND2, HOXC6, PPARG, SORBS2, CD36, and CLDN1). The effects were further supported in additional independent cohorts. Our top hits might play a role in adipogenesis and differentiation, obesity, lipid metabolism, and adipose tissue expandability. Finally, we show that in vitro methylation of SORBS2 directly represses gene expression.

Objective:
Long noncoding RNAs (lncRNAs) are emerging as important regulators of diverse biological processes. Recent work has demonstrated that the inducible lncRNA Blnc1 stimulates thermogenic gene expression during brown and beige adipocyte differentiation. However, whether Blnc1 is functionally conserved in humans has not been explored. In addition, the molecular basis of the Blnc1 ribonucleoprotein complex in thermogenic gene induction remains incompletely understood. The aims of the current study were to: i) investigate functional conservation of Blnc1 in mice and humans and ii) elucidate the molecular mechanisms by which Blnc1 controls the thermogenic gene program in brown adipocytes.

Methods:
Full-length human Blnc1 was cloned and examined for its ability to stimulate brown adipocyte differentiation. Different truncation mutants of Blnc1 were generated to identify functional RNA domains responsible for thermogenic gene induction. RNA-protein interaction studies were performed to delineate the molecular features of the Blnc1 ribonucleoprotein complex.

Results:
Blnc1 is highly conserved in mice and humans at the sequence and function levels, both capable of stimulating brown adipocyte gene expression. A conserved RNA domain was identified to be required and sufficient for the biological activity of Blnc1. We identified hnRNPU as an RNA-binding protein that facilitates the assembly and augments the transcriptional function of the Blnc1/EBF2 ribonucleoprotein complex.

Conclusions:
Blnc1 is a conserved lncRNA that promotes thermogenic gene expression in brown adipocytes through formation of the Blnc1/hnRNPU/EBF2 ribonucleoprotein complex.

Objective:Zfp423 is a multi zinc-finger transcription factor expressed in preadipocytes and mature adipocytes in vivo. Our recent work has revealed a critical role for Zfp423 in maintaining the fate of white adipocytes in adult mice through suppression of the beige cell thermogenic gene program; loss of Zfp423 in mature adipocytes of adult mice results in a white-to-beige phenotypic switch. However, the exact requirements of Zfp423 in the fetal stages of early adipose development in vivo have not been clarified.

Methods:
Here, we utilize two models that confer adipose-specific Zfp423 inactivation during fetal adipose development (Adiponectin-Cre; Zfp423loxP/loxP and Adiponectin-rtTA; TRE-Cre; Zfp423loxP/loxP). We assess the impact of fetal adipose Zfp423 deletion on the initial formation of adipose tissue and evaluate the metabolic consequences of challenging these animals with high-fat diet feeding.

Results:
Deletion of Zfp423 during fetal adipose development results in a different phenotype than is observed when deleting Zfp423 in adipocytes of adult mice. Inactivation of Zfp423 during fetal adipose development results in arrested differentiation, specifically of inguinal white adipocytes, rather than a white-to-beige phenotypic switch that occurs when Zfp423 is inactivated in adult mice. This is likely explained by the observation that adiponectin driven Cre expression is active at an earlier stage of the adipocyte life cycle during fetal subcutaneous adipose development than in adult mice. Upon high-fat diet feeding, obese adipose Zfp423-deficient animals undergo a pathological adipose tissue expansion, associated with ectopic lipid deposition and systemic insulin resistance.

Objective:
Insulin signaling plays a unique role in the regulation of energy homeostasis and the impairment of insulin action is associated with altered lipid metabolism, obesity, and Type 2 Diabetes. The main aim of this study was to provide further insight into the regulatory mechanisms governing the insulin signaling pathway by investigating the role of non-proteolytic ubiquitination in insulin-mediated activation of AKT.

Methods:
The molecular mechanism of AKT regulation through ubiquitination is first dissected in vitro in 3T3-L1 preadipocytes and then validated in vivo using mice with adipo-specific deletion of GPS2, an endogenous inhibitor of Ubc13 activity (GPS2-AKO mice).

Results:
Our results indicate that K63 ubiquitination is a critical component of AKT activation in the insulin signaling pathway and that counter-regulation of this step is provided by GPS2 preventing AKT ubiquitination through inhibition of Ubc13 enzymatic activity. Removal of this negative checkpoint, through GPS2 downregulation or genetic deletion, results in sustained activation of insulin signaling both in vitro and in vivo. As a result, the balance between lipid accumulation and utilization is shifted toward storage in the adipose tissue and GPS2-AKO mice become obese under normal laboratory chow diet. However, the adipose tissue of GPS2-AKO mice is not inflamed, the levels of circulating adiponectin are elevated, and systemic insulin sensitivity is overall improved.

Conclusions:
Our findings characterize a novel layer of regulation of the insulin signaling pathway based on non-proteolytic ubiquitination of AKT and define GPS2 as a previously unrecognized component of the insulin signaling cascade. In accordance with this role, we have shown that GPS2 presence in adipocytes modulates systemic metabolism by restricting the activation of insulin signaling during the fasted state, whereas in absence of GPS2, the adipose tissue is more efficient at lipid storage, and obesity becomes uncoupled from inflammation and insulin resistance.

Objective:
Celastrol was recently identified as a potential novel treatment for obesity. However, the effect of Celastrol on nonalcoholic fatty liver disease (NAFLD) remains elusive. The aim of this study is to evaluate the role of Celastrol in NAFLD.

Conclusions:
Celastrol ameliorates NAFLD by decreasing lipid synthesis and improving the anti-oxidative and anti-inflammatory status. And Sirt1 has an important role in Celastrol-ameliorating liver metabolic damage caused by HFD.

Objective:
Obesity is characterized by excessive fat mass and is associated with serious diseases such as type 2 diabetes. Targeting excess fat mass by sustained lipolysis has been a major challenge for anti-obesity therapies due to unwanted side effects. TLQP-21, a neuropeptide encoded by the pro-peptide VGF (non-acronymic), that binds the complement 3a receptor 1 (C3aR1) on the adipocyte membrane, is emerging as a novel modulator of adipocyte functions and a potential target for obesity-associated diseases. The molecular mechanism is still largely uncharacterized.

Methods:
We used a combination of pharmacological and genetic gain and loss of function approaches. 3T3-L1 and mature murine adipocytes were used for in vitro experiments. Chronic in vivo experiments were conducted on diet-induced obese wild type, β1, β2, β3-adrenergic receptor (AR) deficient and C3aR1 knockout mice. Acute in vivo lipolysis experiments were conducted on Sprague Dawley rats.

Conclusions:
In conclusion, our data identify an alternative pathway modulating lipolysis that could be targeted to diminish fat mass in obesity without the side effects typically observed when using potent pro-lipolytic molecules.

Methods:
We studied glucose homeostasis in lean or obese mice silenced for Endo1 in the ARC via stereotactic injection of shRNA-expressing lentiviral vectors.

Results:
We observed that despite being leaner, Endo1-silenced mice showed impaired glucose homeostasis on HFD. Mechanistically, we show that Endo1 interacts with p85, the regulatory subunit of PI3K, and mediates leptin-induced PI3K activation.

Conclusions:
Our results thus define Endo1 as an important hypothalamic integrator of leptin signaling, and its silencing differentially regulates the OBR-dependent functions.